I don't know if it is too late for me to reply, but I will anyways.
Just like Audrey, from Little Shop of Horrors, Bobby often times has a different, but distinct sound. If they are looking for that specific sound it may be good to sing a song that the character sings in a similar style, like "The Kite" from You're a Good Man, Charlie Brown, or "Suddenly Seymour" from Little Shop of Horrors.
Some other things to look for when picking an audition song are songs that have similar emotion or motivation as the character. An emotional, strong willed song such as "I Believe In You" from How to Suceed in Business Without Really Trying could be compatable to Bobby. Bobby is also comical, so a "ham-it-up" song may be appropriate at a time like this.
Another useful thing when choosing an audition song is looking at the composer. Coming back to the example of Audrey, the same composer, Alan Menken, did The Little Mermaid. Although it could be considered a mainstream audtion song, "Part of Your World" not only is by the same composer, but it has a very similar melody and feeling as "Somewhere That's Green." This strategy doesn't work as well for Urinetown since the composer and lyricist are pretty obscure, but could be helpful in the future.
One other idea is to look up the actor who originated the role to see what other productions he has been in. The original Bobby Strong was Hunter Foster (who happens to be the brother of Sutton Foster). He was Seymour in the new Little Shop of Horrors, and played Leo Bloom on Broadway in The Producers. And just another interesting little fact about Sutton's brother for you fans: he wrote the liberetto for the Off-Broadway musical Summer of '42 which happened to have for the leading lady none other than....Idina Menzel, who was the original Elphaba in Wicked. And... his co-star in Urinetown was Jennifer Laura Thompson who replaced Kristin Chenoweth as Glinda/Galinda on Broadway in Wicked. What a small world! Interesting, no?
The goal in an audition is to sing a song that will remind the director of a specific character. Whether this means singing a song that has a key phrase in it that will lead the director to think of the specific role, a similar melody line, a similar character, or even just a slight tweek in your wardrobe, such as a bow tie. The director needs to find someone to cast in their play. They want you to succeed and be that one. You just need to show them enough to make them say, "Hey, this kid could be a great Bobby Strong." All you need to do is show something that will plant the seed and put you to callbacks for that character. Then you can just blow them away even more when they see you really are perfect for the role. I wish you good luck and I am sure whatever song you sing you will do well! If you have any questions feel free to message me! I love to be able to help!
-I'll give her this much, she doesn't give a twig about what other people think!
-No, she does, she just pretends not to. -Fiyero and Galinda, Wicked
why try to fit in when you are born to stand out?